Biometrics is the science of biological characteristic analysis. Biometric imaging captures a measurable characteristic of a human being for identification of the particular individual (for example, a fingerprint). See, for example, Gary Roethenbaugh, Biometrics Explained, International Computer Security Association, Inc., pp. 1-34 (1998), which is incorporated by reference herein in its entirety.
Traditionally, techniques for obtaining a biometric image have included application of ink to a person's fingertips, for instance, and rolling or simply pressing the tips of the individual's fingers to appropriate places on a recording card. This technique can be very messy due to the application of ink, and may often result in a set of prints that are difficult to read.
Today, biometric image capturing technology includes electro-optical devices for obtaining biometric data from a biometric object, such as, a finger, a palm, etc. In such instances, the electro-optical device may be a fingerprint scanner, a palm scanner, or another type of biometric scanner. These scanners are also referred to as live print scanners. Live print scanners do not require the application of ink to a person's finger or palm. Instead, live print scanners may include a prism located in an optical path. A platen is used as the surface for receiving the biometric object. For example, with an optical fingerprint scanner, a finger is placed on the platen, and a camera detects an image of the fingerprint. The platen can be a surface of the prism or any other surface provided on the prism and in optical contact with the prism. The fingerprint image detected at the camera is comprised of relatively light and dark areas. These areas correspond to the valleys and ridges of the fingerprint.
Live print scanners utilize the optical principle of total internal reflection (TIR). The rays from a light source internal to these optical scanners reach the platen at an incidence angle that causes all of the light rays to be reflected back. This occurs when the angle of incidence is equal to or greater than the critical angle, which is defined at least in part by the ratio of the two indices of refraction of the medium inside and above the surface of the platen.
In the case of a live fingerprint scanner, one or more fingers are placed on the platen for obtaining a fingerprint image. Ridges on a finger operate to alter the refraction index at the platen, thereby interrupting the TIR of the prism. This interruption in the TIR causes an optical image of the ridges and valleys of a fingerprint to be propagated through the receiving surface and captured by a camera internal to the device.
Live fingerprint scanners are increasingly being called upon to operate in a variety of ambient conditions. These conditions can vary in temperature and humidity. Different conditions can affect the quality of a detected image. Also, the particular characteristics of an individual's finger (such as whether it is dry or oily) can affect detected image quality.
For example, in certain cases, the presence of moisture and/or fluids on the finger improves the quality of a detected fingerprint image. Excessive moisture and/or fluids on a finger, however, can be undesirable. Excessive moisture and/or fluids may alter the refraction index at the receiving surface and interrupt the TIR of the prism in undesirable places on the receiving surface. This can degrade image quality. Excessive heat or cold at or near the platen surface can also degrade image quality.
What is needed is a live fingerprint scanner which can operate in a variety of ambient conditions and still capture fingerprint image at a high quality.